Hydraulic systems in a wide variety of forms are indispensable components of many modern work machines. Virtually all tractors, loaders, excavators and other off highway work machines utilize hydraulically actuated work implements. In a typical design, the work machine hydraulic system will include one or more hydraulic pumps coupled with an engine of the work machine and operable to supply a pressurized hydraulic fluid to hydraulic actuators for adjusting the position of a work implement, such as a bucket or blade. In other applications, hydraulic actuators may be used in work machine stabilizers, and even in steering systems of work machines.
Actuation of hydraulic cylinders in the hydraulic system consumes energy provided by the work machine engine, or another power source such as a fuel cell or battery. For example, a boom arm of an excavator or loader will be lifted to a raised position by providing hydraulic fluid to one or more hydraulic actuators coupled therewith. The energy required to raise the boom arm against the force of gravity, for example, may be provided by pressurized hydraulic fluid from the work machine's hydraulic pump. When lowering the boom arm, in contrast, the force of gravity acting on the boom arm will cause it to lower, urging hydraulic fluid out of one side of the hydraulic actuator. In traditional designs, the hydraulic fluid flowing from the actuator is typically transitioned to a low pressure drain. Where the boom arm is in a raised position, the system may be thought of as comprising potential energy, initially inputted to the system in the form of hydraulic energy to raise the boom arm. Where hydraulic pressure is bled to a low pressure drain as the boom arm is lowered, the potential energy residing in the raised boom arm can be lost.
In more modern designs, it is recognized that pressurized fluid derived from a gravity-assisted extension or retraction of a hydraulic actuator may be stored and later returned to the system as needed. In certain designs, an accumulator is connected with the hydraulic system, and can store pressurized hydraulic fluid, then selectively return the fluid to the system on demand. Over the years, engineers have developed a wide variety of designs for recovering hydraulic energy in work machine hydraulic systems. “Hybrid” hydraulic systems, as such designs are referred to in the art, have shown much promise for energy conservation and fuel economy purposes. In certain hybrid designs, fluid may be transitioned directly between the head and rod side of the actuator, rather than sending all of the fluid evacuated from one side of the actuator to drain.
Even the most advanced hybrid hydraulic systems, however, are not without drawbacks. In particular, a phenomenon known in the art as “voiding” may occur in both hybrid and non-hybrid hydraulic systems. Voiding describes the tendency for one of the rod chamber and head chamber of a hydraulic cylinder to develop a void or space, not filled with hydraulic fluid as the actuator is extended or retracted. Voiding is the result at least in part of the difference in fluid volumes between a rod side and a head side of the actuator. In particular, because the rod occupies a certain fluid volume, when fluid is transitioned from the rod side to the head side or a drain, a void may form in the rod side of the actuator. During operation, the generation of such a void may result in a delay in the motion of the hydraulic actuator until the system pumps and/or accumulator can provide the required fluid volume. In certain systems, voiding in the hydraulic pump itself may result from unequal fluid volumes in the head and rod, for example, when extending a hydraulic actuator.
One hydraulic system directed to storing hydraulic energy in a hydraulic system has been described in the technical publication entitled, “Displacement Controlled Linear Actuator With Differential Cylinder—A Way To Save Primary Energy In Mobile Machines”, by Robert Rahmfeld and Monika Ivantysynova of the technical University of Hamberg, Germany. Rahmfeld and Ivantysynova describe a system having a variable displacement, bi-directional hydraulic pump that transitions hydraulic fluid between a head chamber and a rod chamber of a hydraulic cylinder. An accumulator and additional pump are fluidly connected with the hydraulic circuit. While the Rahmfeld and Ivantysynova design offers certain advantages, it also presents certain disadvantages, in particular the fact that much of the hydraulic energy from an overrunning load cannot be stored in the relatively low pressure accumulator.
The present disclosure is directed to overcoming one or more of the problems or shortcomings set forth above.